Garrett's first priority is to change the attitude and effort of the players. He can't tell them, "We play to win and you need to prove you are giving your maximum effort to win", and then turn around and start treating the rest of the season like it is a try-out for next-year's players.
While I agree that McGee is more mobile than Kitna, there is no way he can read a defense or anticipate a blitz or route like an experienced QB like Kitna.
Winning and losing are both habit-forming and attitude multipliers. I see a lot of posters saying, "let's use the rest of this season as an extended pre-season for next year so we can evaluate the young players on the roster and get a top five draft pick." Go ahead, lose 12-13 in a row and see how tough it is to have a positive attitude and effort when you decide it's time to want to play to win.
"Winning isn't the most important thing, it's the only thing" (to quote an arch-enemy).
I believe that McGee can play QB in the NFL, but I don't believe he's ready. Kitna gives the team the best chance to win right now. And a lack of focus on "right now" is what put this team where it is.